Photodynamic-Immune Checkpoint Therapy Eradicates Local and Distant Tumors by CD8⁺ T Cells

Photodynamic therapy (PDT) is a clinically applied tumor ablation method that reduces tumor burden and may induce T-cell responses, providing a therapeutic option for mutated tumors. In this study, we applied PDT in two mouse tumor models and assessed its effect on outgrowth of PDT-treated and distant untreated tumors. PDT of established tumors resulted in complete tumor eradication in most mice, which were then protected against tumor rechallenge. Correspondingly, the therapeutic effect was abrogated upon systemic depletion of CD8⁺ T cells, indicating PDT-induced tumor antigen cross-presentation and T-cell activation. In a double-tumor model, PDT of primary tumors induced enhanced infiltration of untreated distant tumors by CD8⁺ T cells, which significantly delayed their outgrowth. Combination therapy of PDT and CTLA-4-blocking antibodies significantly improved therapeutic efficacy and survival of double-tumor-bearing mice. These results show that local tumor ablation by PDT induces CD8⁺ T-cell responses crucial for systemic tumor eradication, which can be further enhanced by combination with immune checkpoint blockade. This combination of two clinically applied therapies may be a treatment strategy for advanced cancer without previous knowledge of tumor-specific antigens. Cancer Immunol Res; 5(10); 832-8. ©2017 AACR.